1. Field of the Invention
This invention relates to nuclear magnetic resonance (NMR) spectroscopy and, in particular, to probes used in high resolution NMR experiments.
2. Description of the Related Art
All atomic nuclei of elements with an odd atomic mass or an odd atomic number possess a nuclear magnetic moment. Nuclear magnetic resonance is a phenomenon exhibited by this select group of atomic nuclei (termed "NMR active" nuclei), and is based upon the interaction of the nucleus with an applied, external magnetic field. The magnetic properties of a nucleus are conveniently discussed in terms of two quantities: the gyromagnetic ratio (y); and the nuclear spin (I). When an NMR active nucleus is placed in a magnetic field, its nuclear magnetic energy levels are split in to (21+1) non-degenerate energy levels, which are separated from each other by an energy difference that is directly proportional to the strength of the applied magnetic field. This splitting is called the "Zeeman" splitting and the energy difference is equal to hH.sub.o /2.pi., where h is Plank's constant and H.sub.o is the strength of the applied magnetic field. The frequency corresponding to the energy of the Zeeman splitting (.omega..sub.o =yH.sub.o) is called the "Larmor frequency" and is proportional to the field strength of the magnetic field. Typical NMR active nuclei include .sup.1 H (protons), .sup.13 C, .sup.19 F, .sup.31 P. For these four nuclei I=1/2, and each nucleus has two nuclear magnetic energy levels.
When a bulk sample of material containing NMR active nuclei is placed within a magnetic field called the main static field, the nuclear spins distribute themselves among the nuclear magnetic energy levels in accordance with Boltzmann's statistics. This results in a population imbalance among the energy levels and a net nuclear magnetization. It is this net nuclear magnetization that is studied by NMR techniques.
At equilibrium, the net nuclear magnetization of the aforementioned bulk sample is aligned parallel to the external magnetic field and is static (by convention, the direction of the main static field is taken to be the z-axis). A second magnetic field perpendicular to the main static magnetic field and rotating at, or near, the Larmor frequency can be applied to induce a coherent motion of the net nuclear magnetization. Since, at conventional main static magnetic field strengths, the Larmor frequency is in the megahertz frequency range, this second magnetic field is called a "radio frequency" or RF field.
The effect of the RF field is to shift the nuclear magnetization direction so that it is no longer parallel to the main static field. This shift introduces a net coherent motion of the nuclear magnetization about the main static field direction called a "nutation". In order to conveniently deal with this nutation, a reference frame is used which rotates about the laboratory reference frame z-axis at the Larmor frequency and also has its z-axis parallel to the main static field direction. In this "rotating frame" the net nuclear magnetization, which is rotating in the stationary "laboratory" reference frame, is now static.
Consequently, the effect of the RF field is to rotate the now static nuclear magnetization direction at an angle with respect to the main static field direction (z-axis). The new magnetization direction can be broken into a component which is parallel to the main field direction (z-axis direction) and a component which lies in the plane transverse to the main magnetization (x, y plane). The RF field is typically applied in pulses of varying length and amplitude and, by convention, an RF pulse of sufficient amplitude and length to rotate the nuclear magnetization in the rotating frame through an angle of 90.degree., or .pi./2 radians, and entirely into the x, y plane is called a ".pi./2 pulse".
Because the net nuclear magnetization is rotating with respect to the laboratory reference frame, the component of the nuclear magnetization that is transverse to the main magnetic field, or that lies in the x, y plane, rotates about the external magnetic field at the Larmor frequency. This rotation can be detected with a receiver coil that is resonant at the Larmor frequency. The receiver coil is generally located so that it senses voltage changes along one axis (for example, the x-axis) where the rotating magnetization component appears as an oscillating voltage. Frequently, the "transmitter coil" employed for applying the RF field to the sample and the "receiver coil" employed for detecting the magnetization are one and the same coil.
Although the main static field is applied to the overall material sample, the nuclear magnetic moment in each nucleus within the sample actually experiences an external magnetic field that is changed from the main static field value due to a screening from the surrounding electron cloud. This screening results in a slight shift in the Larmor frequency for that nucleus (called the "chemical shift" since the size and symmetry of the shielding effect is dependent on the chemical composition of the sample).
In a typical NMR experiment, the sample is placed in the main static field and a .pi./2 pulse is applied to shift the net magnetization into the transverse plane (called transverse magnetization). After application of the pulse, the transverse magnetization, or "coherence", begins to precess about the x-axis, or "evolve," due to the chemical shifts at a frequency which is proportional to the chemical shift field strength. In the rotating frame, the detector (which is stationary in the laboratory frame) appears to rotate at the Larmor frequency. Consequently, the detector senses an oscillation produced by an apparent magnetization rotation at a frequency which is proportional to the frequency difference between the Larmor frequency and the chemical shift frequency. Thus, the detected signal oscillates at the frequency shift difference.
In addition to precessing at the Larmor frequency, in the absence of the applied RF field energy, the nuclear magnetization also undergoes two spontaneous processes: (1) the precessions of various individual nuclear spins which generate the net nuclear magnetization become dephased with respect to each other so that the magnetization within the transverse plane loses phase coherence (so-called "spin-spin relaxation") with an associated relaxation time, T.sub.2 ; and (2) the individual nuclear spins return to their equilibrium population of the nuclear magnetic energy levels (so-called "spin-lattice relaxation") with an associated relaxation time, T.sub.1. The latter process causes the received signal to decay to zero. The decaying, oscillating signal is called a free induction decay (FID).
This invention relates to a phenomenon observed in NMR experiments of samples with strong signals. The amount of signal that is detected with the receiver coil of an NMR probe is proportional to the amount of magnetization from the sample and to the quality factor Q of the probe circuit electronics. In the case of a sample with a high concentration of similar spins, such as in proton NMR of water in a probe with a high Q value, a strong NMR signal is detected. The large proton magnetization induces a large current in the receiver coil and this current, in turn, acts as an RF pulse, generating an oscillating magnetic field of the same frequency. This phenomenon is referred to as radiation damping. As a result of radiation damping, the magnetization, rotated by an RF pulse into the x, y plane, is rotated back to the positive z-axis by the radiation field that is induced by its own NMR signal. Radiation damping thus limits the time that the magnetization spends in the x, y plane and, consequently, limits the time available for signal detection. As a result, the resonance lines of spins affected by radiation damping are broadened which results in unwanted overlap of resonances and loss in spectral resolution.
Radiation damping decreases the observable free induction decay by reducing the amount of time which the magnetization spends in the transverse plane. This, in turn, leads to a broadening of the resonance line obtained after a Fourier transformation of the free induction decay. The rate of change of the magnetization vector due to radiation damping can be characterized by a time constant T.sub.d, which depends on the amount of magnetization, the gyromagnetic constant y of the spins, and the quality factor Q of the probe. This rate of change is described by the equation: EQU T.sub.d.sup.-1 =2.pi.QnyM.sub.o
where M.sub.o is the equilibrium magnetization and n is the filling factor of the coil
A prior art approach to reduce radiation damping is to lower the Q of the probe circuit. This technique is described in detail in a publication entitled "Principles of Nuclear Magnetism", A. Abragam, International Series of Monographs on Physics, volume 32, Clarendon press, Oxford, 1989. By lowering the quality factor Q of the circuit, less voltage is induced in the receiver coil, resulting in a smaller current and a corresponding reduction in radiation damping. However, this method causes a loss in signal intensity since the induced voltage is proportional to the Q of the circuit. Furthermore, to obtain a 90.degree. rotation of the magnetization, this method requires longer RF pulse lengths than are typically used with a high Q circuit.
The probes that are routinely used for NMR experiments have a high Q value to provide optimum sensitivity. The lowering of the Q value in these probes is achieved by manually detuning the probe electronics. The extent to which the Q is lowered is a compromise between a loss in signal strength and RF field strength and the suppression of radiation damping, a balance point that has been determined by trial and error. While this compromise helps reduce the detrimental effects of radiation damping, the accompanying loss in signal strength and RF field strength greatly decreases the sensitivity of the probe.
In another prior art method, the reduction of the quality factor of a probe circuit is used in a two-dimensional resonance experiment. In such experiments, an evolution time is used between the time of an excitation pulse and a subsequent mixing pulse, after which two dimensional data sampling is performed. Such a two-dimensional technique measures the correlation of spin dynamics in the two time intervals. For this type of experiment, it is known to reduce the Q of the probe circuit during the evolution time to reduce radiation damping which would otherwise increase the effect of large magnitude resonance signals (notably that of water) on the measured output. However, in this method, the Q reduction occurs only during the evolution time.
In still another prior art method, active Q switching is used to toggle between two nominally orthogonal RF coils during imaging of a solid sample. This method involves alternately switching low the Q of a circuit driving a quadrupolar NMR coil (which generates a gradient field) and a circuit driving a linear NMR coil (which generates a homogeneous field). The alternating Q switching results in the circuit for the quadrupolar coil having a high Q when the circuit for the linear coil has a low Q, and vice versa. This prior art method involves a combination of multiple pulse line narrowing and RF gradient spatial encoding both of which occur during data collection. The line narrowing is performed via the homogeneous coil with the homogeneous coil's Q high, and the encoding is performed via the gradient coil with the gradient coil's Q high. By toggling the high-Q state between the two coil circuits, the line narrowing, spatial encoding and data acquisition are intercolated into a periodic sequence. This method is unconcerned with the problems of radiation damping in sharp line resonance measurements.
It is therefore an object of the present invention to provide a means for reducing radiation damping during data acquisition in sharp line NMR experiments in a manner which is both reproducible and predictable, without being subjected to the magnitude of signal loss typical of prior art systems.